How to Bind to Property Which Resides in the App Xaml backend

I have read here that you can't have a dependency property on the App class.

Indeed you can't, because Application doesn't inherit from DependencyObject. However, it's not necessary : only the target property of a binding needs to be a dependency property.

If you want to bind to a property of your App class, you can do it like that :

IsEnabled="{Binding Path=User, Source={x:Static Application.Current}}"

And to access it programatically in C# you can do this:

((App)Application.Current).YourMethod
((App)Application.Current).YourProperty